[PATCH v3 2/5] splice: Provide pipe_head_buf() helper

[David Howells <dhowells@redhat.com>]

Provide a helper, pipe_head_buf(), to get the current head buffer from a
pipe.  Implement this as a wrapper around a more general function,
pipe_buf(), that gets a specified buffer.

Requested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
cc: Jens Axboe <axboe@kernel.dk>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: John Hubbard <jhubbard@nvidia.com>
cc: David Hildenbrand <david@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-block@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
---
 fs/splice.c               |  9 +++------
 include/linux/pipe_fs_i.h | 20 ++++++++++++++++++++
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index 91b9e2cb9e03..7c0ff187f87a 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -295,7 +295,6 @@ static ssize_t direct_splice_read(struct file *in, loff_t *ppos,
 	struct bio_vec *bv;
 	struct kiocb kiocb;
 	struct page **pages;
-	unsigned int head;
 	ssize_t ret;
 	size_t used, npages, chunk, remain, reclaim;
 	int i;
@@ -358,9 +357,8 @@ static ssize_t direct_splice_read(struct file *in, loff_t *ppos,
 	}
 
 	/* Push the remaining pages into the pipe. */
-	head = pipe->head;
 	for (i = 0; i < npages; i++) {
-		struct pipe_buffer *buf = &pipe->bufs[head & (pipe->ring_size - 1)];
+		struct pipe_buffer *buf = pipe_head_buf(pipe);
 
 		chunk = min_t(size_t, remain, PAGE_SIZE);
 		*buf = (struct pipe_buffer) {
@@ -369,10 +367,9 @@ static ssize_t direct_splice_read(struct file *in, loff_t *ppos,
 			.offset	= 0,
 			.len	= chunk,
 		};
-		head++;
+		pipe->head++;
 		remain -= chunk;
 	}
-	pipe->head = head;
 
 	kfree(bv);
 	return ret;
@@ -394,7 +391,7 @@ static size_t splice_folio_into_pipe(struct pipe_inode_info *pipe,
 
 	while (spliced < size &&
 	       !pipe_full(pipe->head, pipe->tail, pipe->max_usage)) {
-		struct pipe_buffer *buf = &pipe->bufs[pipe->head & (pipe->ring_size - 1)];
+		struct pipe_buffer *buf = pipe_head_buf(pipe);
 		size_t part = min_t(size_t, PAGE_SIZE - offset, size - spliced);
 
 		*buf = (struct pipe_buffer) {
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 6cb65df3e3ba..d2c3f16cf6b1 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -156,6 +156,26 @@ static inline bool pipe_full(unsigned int head, unsigned int tail,
 	return pipe_occupancy(head, tail) >= limit;
 }
 
+/**
+ * pipe_buf - Return the pipe buffer for the specified slot in the pipe ring
+ * @pipe: The pipe to access
+ * @slot: The slot of interest
+ */
+static inline struct pipe_buffer *pipe_buf(const struct pipe_inode_info *pipe,
+					   unsigned int slot)
+{
+	return &pipe->bufs[slot & (pipe->ring_size - 1)];
+}
+
+/**
+ * pipe_head_buf - Return the pipe buffer at the head of the pipe ring
+ * @pipe: The pipe to access
+ */
+static inline struct pipe_buffer *pipe_head_buf(const struct pipe_inode_info *pipe)
+{
+	return pipe_buf(pipe, pipe->head);
+}
+
 /**
  * pipe_buf_get - get a reference to a pipe_buffer
  * @pipe:	the pipe that the buffer belongs to